Internal combustion engines must meet various regulations for reduced emissions and improved fuel economy. One example of a way to improve fuel economy is to operate an engine at an air/fuel ratio that is lean (excess oxygen) of stoichiometry. Examples of lean-burn engines include compression-ignition (diesel) and lean-burn spark-ignition engines. While a lean-burn engine has improved fuel economy, and lower combustion temperatures, which generally result in increased engine-out nitrogen oxides (NOX) emissions, commercial application of lean-burn engines is limited due to a lack of effective methods to remove sufficient NOX from the lean exhaust stream before it exits the tail pipe to meet regulations.
Reduction of NOX emissions from an exhaust stream including excess oxygen is a challenge for vehicle manufacturers. It is estimated that compliance with Bin 5 regulations in the United States may require an aftertreatment system capable of 70-90% NOX conversion efficiency on the FTP (Federal Test Procedure) cycle based on currently anticipated engine-out NOX levels at a variety of operating temperatures ranging between 200-550° C.
Various aftertreatment systems have been proposed for vehicle applications that employ various exhaust aftertreatment devices. Urea selective catalyst reduction (SCR) catalyst devices employ a NOX reductant, e.g., urea, that is injected upstream of the catalyst and is converted to ammonia for reduction of NOX to N2. Use of urea as a reductant necessitates a urea distribution infrastructure and an on-vehicle monitoring system for this secondary fluid, and may require thermal management to address potential problems in cold weather climates due to the relatively high freezing point (−12° C.) of the urea solution. NOX storage SCR catalysts, e.g., NOX traps, typically require large catalyst volumes, large amounts of platinum-group metals and low sulfur fuel for efficient storage operation. Such systems require periodic catalyst regeneration involving fuel injection to generate high exhaust gas temperatures and injection of reductants to regenerate the storage material of the catalyst.
While systems that employ SCR catalysts have been used for NOX reduction in exhaust gas flow streams having excess oxygen, packaging of the various catalysts has been problematic, particularly in relatively smaller vehicles having relatively shorter wheelbases, due the reduced space available to package the desired combinations of catalysts. For example, in some smaller vehicles, it is desirable to package the SCR last where it is farthest from the engine and the exhaust system operating temperatures are lowest, in order to minimize thermal degradation of the SCR catalyst materials and thereby maximize the operating life of the SCR catalyst. While this arrangement is desirable, there is generally not enough room to package the SCR last while also providing the needed mixing length for conversion of the injected urea into ammonia, particularly if the system also employs one or more additional exhaust treatment devices for the reduction of NOX or oxidation or reduction of other exhaust constituents, including carbon monoxide (CO), various hydrocarbons (HC), particulate matter (PM) and the like. Even where treatment devices are placed away from the engine to lower their operating temperature and improve the device operating life, there are frequently competing considerations, such as device regeneration, that require periodically elevating the device temperature by reheating, which generally employs fuel and reduces the engine and vehicle efficiency and reduces fuel economy. In general, exhaust treatment systems must balance these competing considerations and comply with applicable emission control requirements, particularly the reduction of NOX, while also complying with applicable fuel economy and other engine and/or vehicle requirements.
Accordingly, it is desirable to provide exhaust treatment systems for internal combustion engines, particularly vehicular engines, which provide enhanced flexibility to satisfy competing requirements, including those related to NOX reduction, fuel economy, thermal management, system/device operating longevity and the like.